The present application relates to an optical device utilizing an electrowetting phenomenon, and a housing constituting such an optical device.
In recent years, optical devices utilizing an electrowetting phenomenon (electrocapillarity) have been under development. The term “electrowetting phenomenon” refers to a phenomenon in which, when a voltage is applied between a conductive liquid and an electrode, energy at the solid-liquid interface between the surface of the electrode and the liquid changes, resulting in a change in the shape of the surface of the liquid.
FIGS. 6A and 6B are views illustrating the principle of the electrowetting phenomenon. As schematically shown in FIG. 6A, for example, it is assumed that an insulating layer 102 is disposed on the surface of an electrode 101, and a conductive droplet 103 of an electrolytic solution is placed on the insulating layer 102. The surface of the insulating layer 102 has been subjected to water repellency treatment. As shown in FIG. 6A, in a state where a voltage is not applied, the interaction energy between the surface of the insulating layer 102 and the droplet 103 is low, and the contact angle θ0 is large. The contact angle θ0 is defined as the angle between the surface of the insulating layer 102 and the tangent line to the droplet 103, and depends on physical properties, such as surface tension of the droplet 103 and surface energy of the insulating layer 102.
On the other hand, as schematically shown in FIG. 6B, in a state where a voltage is applied between the electrode 101 and the droplet 103, electrolyte ions of the droplet concentrate near the surface of the insulating layer 102, resulting in a change in the amount of charge in the charge double layer, which induces a change in the surface tension of the droplet 103. This is the electrowetting phenomenon, and the contact angle θV of the droplet 103 changes depending on the amount of applied voltage. That is, in FIG. 6B, the contact angle θV can be expressed as a function of the applied voltage V according to the Lippman-Young equation (A) below.cos(θV)=cos(θ0)+(½)(∈0·∈)/(γLG·t)×V2  (A)
where ∈0 is the dielectric constant of the vacuum, ∈ is the relative dielectric constant of the insulating layer, γLG is the surface tension of the electrolytic solution, and t is the thickness of the insulating layer.
As described above, the surface shape (curvature) of the droplet 103 changes depending on the amount of the voltage V applied between the electrode 101 and the droplet 103. Consequently, when the droplet 103 is used as a lens element, it is possible to realize an optical element in which the focal position (focal length) can be electrically controlled.
Optical devices using such an optical element have been under development. For example, Japanese Unexamined Patent Application Publication No. 2000-356708 proposes a lens array for a stroboscopic device. In the lens array, droplets of an insulating liquid which are disposed in an array on a water-repellent film on the surface of a substrate and a conductive liquid are encapsulated to constitute variable-focus lenses. In this structure, individual lenses are formed in the shape of the interface between the insulating liquid and the conductive liquid. The shape of each lens is electrically controlled by utilizing the electrowetting phenomenon so as to change the focal length. Furthermore, Japanese Unexamined Patent Application Publication No. 2002-162507 discloses a cylindrical lens constituted by a liquid lens.